1. Field of the Invention
The present invention relates to a signal processing system which uses Cartesian loop control. In particular, the present invention relates to a system which may be used for linearising a power amplifier for use in, for example, a radio transmitter which, in turn, may be used in, for example, a VHF digital air traffic control communications system.
2. Discussion of the Background
Many different signal processing systems which are capable of producing output signals and controlling downstream components or systems in an accurate fashion are known in the art. One class of such signal processing systems is that of feedback control systems.
One particular method of feedback control is that of Cartesian feedback control, wherein a control input is split into orthogonal componentsxe2x80x94an in-phase and a quadrature componentxe2x80x94and a feedback signal is similarly split. Feedback is then carried out using these split control input and feedback signals.
A particular application for such Cartesian feedback control is in the linearisation of amplifier stages.
A particular application of linearised amplifier stages is in radio transmitters.
For ease of understanding, the present invention will be described in terms of a particular embodiment in the area of radio transmitters, though its application to many and varied other areas of technology would be evident to a person skilled in the art.
FIG. 1 shows a schematic drawing of a conventional radio transmitter. A signal to be transmitted is created at block 1xe2x80x94via a microphone, for example; this signal is then processed in block 2xe2x80x94modulated, converted to an appropriate frequency for transmission etc.; the processed signal is then amplified (3) and transmitted via some form of antenna (4).
In various applications, the purity of the spectrum has to be very high. This is especially true for applications where a fixed waveband has been allocated for a particular use and there is a desire to get as many users into the available waveband as possible. Such applications include, for example: air traffic control, mobile telephones, radio and television station broadcasts.
For certain modulation standards, the requirement for high spectral purity implies a need for an accurate linear amplification stage (3) in the transmitter. Three options appear to suggest themselves for adequately linearising a transmitter power amplifier:
(i) The use of a very large power amplifier running well below its maximum power and thus well away from non-linear regions. This is the simplest option, but is very inefficient and is a relatively cumbersome optionxe2x80x94inevitably large and expensive to make and run.
(ii) The use of a feedforward control technique in which an error signal is formed from the output of the amplifier, by comparison with the input, and the error signal is fed forward and combined in a coupler with the amplifier output to reduce distortion. This technique does allow the use of power-efficient amplifiers, but, it has associated problems. The circuitry of a feedforward technique circuit is sensitive to requirements for the matching of components in the various loops and in transmitter/receiver matching requirements. Such circuits are thus dependent on component characteristics, such as temperature dependence etc. Thus there are practical limits to the degree of spectral control which can be achieved using this feedforward technique and there is substantial cost involved in achieving adequate component matching.
(iii)The use of the Cartesian loop technique, which, in its conventional form (see, for example, IEE Conference Publication Number 235, pages 161-165; V. Petrovic: xe2x80x9cVHF SSB Transmitter employing Cartesian Feedbackxe2x80x9d), allows the use of a power-efficient amplifier and provides very accurate spectral control without the particular matching requirements of the feedforward technique.
However, Cartesian loop control requires control of phase shift around the loop, and this introduces problems with tuning the transmitter over a band of frequencies. In Cartesian controllers, the transmitter has to be readjusted each time the transmission frequency is changed. Prior art controllers used phase-shifting networks operating at radio frequency (RF). These RF phase-shift networks are difficult to design and limited in their phase control range, frequently making it necessary to introduce phase control at more than one point in the circuit. Furthermore, they are subject to drift with time and temperature. If a transmitter is to cover anything other than an extremely narrow band of frequencies, the RF phase-shifting networks need to be readjusted each time a frequency change is made. This is either done manually, which is time consuming and requires skilled personnel, or automatically controlled, e.g. by a microprocessor, which places further demands on the RF phase-shifting network.
A conventional Cartesian feedback apparatus, such as that described in Petrovic""s paper (supra), is shown in FIG. 2. In such conventional Cartesian feedback apparatus, the error signals (Ierr, Qerr) are used directly to control the magnitude of the I and Q components derived from the VCO (Voltage Controlled Oscillator) and thus control the magnitude and phase of the transmitter output.
Conventional Cartesian feedback methods use a single VCO for xe2x80x9cconverting upxe2x80x9d signals to radio frequency (RF) as shown in FIG. 3 (see also FIG. 3 of IEE Conference Publication Number 235, pages 161-165; V. Petrovic: xe2x80x9cVHF SSB Transmitter employing Cartesian Feedbackxe2x80x9d).
WO-A-96/37948 (British Technology Group) discloses a method for producing a linear transmitter. In this method, the output of the circuit is provided by the control of the two separate oscillators such that their sum is the desired output signal. These separate VCOs will need to be accurately matched and thus such a system will be incapable of the desired accuracy unless a Cartesian feedback is also used. Such a system does not appear to effectively overcome the limitation of not being able to produce a Cartesian feedback loop system which can operate over a broad band of operating frequencies.
An object of the present invention is to provide a signal processing system and method which very accurately control a downstream component or system over a wide band of frequencies, using a feedback technique which has the advantages of Cartesian loop control mentioned above and which does not have the difficult component-matching requirements of some of the techniques mentioned above.
In one aspect of the invention, such a system is provided by a signal processing system which comprises:
at least one control loop having a forward path and a feedback path;
signal processing means for producing a representation of the desired downstream output amplitude and phase as a pair of substantially orthogonal signal components;
feedback-path splitting means for splitting a feedback signal into a plurality of substantially orthogonal feedback signal components;
first combining means for combining the substantially orthogonal feedback signal components with the corresponding substantially orthogonal signal components to create error signals;
a plurality of modulating means which control the amplitude, frequency and/or phase of the pair of substantially orthogonal signal components on the basis of control signals which are derived from the error signals; and
second combining means for combining the outputs of said plurality of modulating means into an output signal;
wherein, said signal processing system further comprises a phase-shift control means in the forward path of the control loop which derives said control signals from said error signals on the basis of monitored signal values from at least one predetermined point in the signal processing system.
Such a system has the advantages that it provides the very high accuracy of Cartesian feedback control for the control of downline components or systems whilst being able to operate over a wide band of operating frequencies and that it does not have the difficult component-matching requirements or transmitter/receiver matching requirements of some other systems. This latter advantage, in turn, means that the system of the invention is easily repeatablexe2x80x94it may be produced in large quantities without the need for extended set-up and trimming procedures which are both time-consuming and costly. Using such a set-up, the phase may be adjusted at willxe2x80x94anywhere between 0xc2x0 and 360xc2x0.
Some other aspects and advantages of the invention are as follows:
the second phase-shift control means may operate at baseband frequencyxe2x80x94allowing the use of simple, repeatable and cheap components;
the phase-shift control means may be digital;
the phase-shift control means may be digitally controlled;
the phase-shift control means may be auto-calibrated. This could be achieved in situations where an initialising input signal sequence (a xe2x80x9cpreamblexe2x80x9d) is known and the phase-shift control means can check what the feedback is for this known sequence, then, if the feedback is wrong, it can be automatically trimmed accordingly;
if the phase-shift control means is digitally controlled, it may be made self-calibrating using the same principle as for auto-calibration and running a simple algorithm in the digital control means in order to self-trim the phase-shift control means.
The phase-shift control means can also be used to control the gain of the control loop and thus be used to compensate for variable gains within other loop components.